1. Field of the Invention
The present invention relates to a method of manufacturing an electric heating device which is in particular employed as auxiliary heater in a motor vehicle. The electric heating device comprises a housing in which a heating block is held with pretensioning force of at least one spring element located approximately at the same level as the heating block in its post assembly position. The housing furthermore embodies opposite housing openings between which the heating block is exposed.
2. Description of the Related Art
Such an auxiliary heater for air-conditioning the passenger compartment of a motor vehicle is known, for example, from EP 1 564 503. In the prior method of manufacturing the electric heating device, the elements forming the heating block are introduced into a housing part. Furthermore, the spring element is introduced into the housing part. Then, the housing is closed by placing another housing part onto the first housing part. According to the teaching of EP 1 564 503, the spring element is not pretensioned before the housing is closed, the pretensioning being effected by displacement elements which are formed at the other housing part and which pretension the spring element. These displacement elements form an inclined plane past which the spring element slides with its upper exposed limb and is in the process pivoted towards the heating block about a bearing within the first housing part and is finally pretensioned for generating a tension force.
The heating block of the generic electric heating device usually comprises several parallel layers of heat-dissipating and heat generating elements. The heat generating elements of the heating block comprise several PTC heating elements, which are provided overlapping in one plane and are arranged between printed conductors, which are normally formed by sheet metal bands. These printed conductors carry current with different polarities. The PTC elements can be glued to these printed conductors. It is also possible for the printed conductors to contact the PTC heating elements under tension. In any case it must be ensured that for extracting the heat produced by the PTC heating elements and for feeding current, good contact between the printed conductors and the PTC heating elements exists.
One or more heat generating elements can be provided as part of the heating block. The heat produced by the heat generating elements is dissipated through heat dissipating elements to the medium to be heated, i.e. the air. It flows through the housing through the two frame openings, which accommodate the flat heating block. The frame openings here lie normally parallel to one another on oppositely situated sides of an essentially flat, frame-shaped housing. With regard to the most economical manufacture of the electrical heating device, the heat dissipating elements are generally formed from meander-type bent sheet strips, which form corrugated ribs. These corrugated ribs contact heat dissipating elements on one or both sides. Consequently, the heating block comprises several layers of heat dissipating and heat generating elements, wherein it must also be ensured with regard to the thermal emission that the heat dissipating elements have a good contact to the heat generating elements. Also in this respect, the heat dissipating elements can be permanently joined to the heat generating elements and/or contact them under tension through at least one spring element accommodated in the housing.
Instead of a meander-type sheet metal band, the heat dissipating element can also be formed by an extruded aluminium profile, which forms ridges, which extend essentially at right angles to the layers of the layer structure comprising the heat dissipating and the heat generating elements. In a case of this nature the printed conductor, i.e. the generally flat locating face, for the PTC heating element can be formed by the outer surface of an extruded aluminium profile of this nature. With both alternatives, corrugated rib element or extruded profile, the locating face for the PTC heating elements is formed electrically conducting and is electrically connected to contacts which are normally mounted insulated from one another. In the first case the contacts are generally formed by the exposed ends of the sheet metal bands.
The layered heating block consisting of parallel heat dissipating and heat generating elements, optionally with one or more additional spring elements extending parallel to it, is preferably mounted in a housing with a U-shaped cross-section. When the layer structure is subjected to the pressure of a spring, the frame has to be dimensioned such that the spring force can be continuously maintained even at increased temperatures. Here it should be noted that the insulating frame is nowadays manufactured as an injection moulded part, due partially to economical reasons. Normal housings nowadays consist of a housing lower part and a housing upper part. The housing lower part here forms a receptacle for the individual elements of the heating block and, where required, of the spring element. The individual elements of the heating block are arranged in this housing lower part. Then the heating block is enclosed in the housing by joining the housing upper part and the housing lower part. To achieve this, edges which surround the frame openings can partially cover the heating block so that the heating block is enclosed between the frame openings and mounted in the housing. The two housing parts are then joined together, for example using a latching connection.
With this type of assembly there is the problem that the individual layers of the heating block must be arranged at a predetermined point in the housing. Since not every heat generating element is assigned its own contacts, the electrical conditions within the heating block must also be considered during assembly. However to minimise the manufacturing costs, there is also the desire to form the parts of the heating block as standardised as possible, so that identical components can be used for different layers of the heating block.
Furthermore, the housing itself should be able to be manufactured as simply as possible with regard to an economical manufacture of the electrical heating device. Here however, the particular requirements for the practical installation of one or more spring elements in the housing have to be followed when on joining the housing parts the heating block is already subjected to prestressing in the frame so that joining has to take place against this prestressing.
With regard to the previously discussed problems, in EP 1 564 503 an electrical heating device of the generic type has already been suggested in which the layers of the heating block including a spring element are first mounted in a housing lower part initially free of tension. A housing upper part, which can be connected to it, forms an oblique sliding surface which extends over the end of the spring element protruding up from the housing lower part in relation to the outer side of the heating block. When joining the housing upper part and housing lower part, the spring element is accordingly compressed in the direction of the heating block and contacts it prestressed.
This prior suggestion leads to a certain simplification during assembly, which however requires that the elements of the heating block, as also the spring element, are brought into the correct positions in the housing lower part. Furthermore, the housing implemented with this electrical heating device has various oblique surfaces, which are required for stressing and enclosing the spring element when joining the housing parts. Moreover, the embodiment of the spring element has to be relatively complex to fulfill the task.
The present invention is based on the problem to provide a method of manufacturing an electric heating device, which allows a more simple assembly of the elements of the electric heating device. Furthermore, the present invention wishes to provide an electric heating device for carrying out the method which can be produced in a simpler and thus cheaper manner.